I think this is Millard Canyon below the waterfall, and they're looking up at someone at the top of the falls. (Or perhaps they're wondering where the water's got to - soon they'll find out a water company accidentally diverted it all).

It is 1911 and while it might seem like this is an early Pasadena outing of the newly formed Angeles Chapter of the Sierra Club, that would be incorrect. It actually is an early Pasadena outing of the newly formed Audubon Society. That is why people are looking up into the trees. It is not Eaton Canyon or the Arroyo Seco. It is Rubio Canyon, just below the lower falls (which sadly were obliterated by a misplaced explosion commissioned by the Las Flores Water Company which destroyed all six water falls in the canyon in 1999).In the lower right of the picture, standing on a small boulder, wearing a hat, is a young Victor Fleming. Born in Pasadena, Fleming went on to direct the motion pictures, Wizard of Oz and Gone With The Wind, for which he won an academy award.